The Core Themes Directory
When starting out, one of the first things people do is navigate to the core
/themes directory and take a look at the files in the themes to get an idea of the general structure and contents. Unfortunately, many people make the mistake of starting out by directly customizing core themes. Do not make this mistake! They usually run into roadblocks and frustration shortly thereafter. Drupal has a large and diverse user base, and the main goal of a core Drupal theme is to cater to the masses.
Aesthetics aside, core themes have many requirements and different use cases to satisfy. A few themes support the Color module in order to make it easy for site administrators to change color schemes in the user interface. This is not a bad thing; however, it can easily become confusing and frustrating when trying to customize colorized themes because CSS is generated programmatically and stored outside of the theme directory. Core themes must also function if used as an administration theme and they must support bidirectional text; in general, they can’t stray far from Drupal’s default regions and settings.
It’s not easy to please everyone, and Drupal core themes have the tough job of trying to do just that. As a result, core themes are nowhere near as flexible or as cutting edge as they could be. Most of the time, your goal and approach will be very different when creating custom themes. You’ll be able to focus on coding your own front-end or back-end focused design, customize the markup, decide which CSS files to use (if any), and other exciting decisions.